The present invention relates to a method for forming a protective coating on a turbine engine component having an external surface and an internal cavity defined by an internal surface connected to the external surface by at least one hole. More particularly, the invention relates to forming an aluminide coating on the internal and external surfaces of a turbine component, such as a turbine blade having an internal cooling cavity.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. (As used herein, the term turbine blade may refer to either a turbine blade or a turbine vane, which have similar appearance in pertinent portions.) The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine blades. In current engines, the turbine blades are made of nickel-based superalloys, and can operate at metal temperatures of up to about 1900-2100° F. (about 1038-1149° C.).
Turbine blades typically comprise cooling circuits that channel cooling air through the interior of the turbine airfoil to reduce temperatures encountered by the blade and improve part life. During operation of the jet engine, air is forced through the root portion of the blade, into the airfoil cooling chambers, and out openings at the external surface of the airfoil. The flow of the air removes heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil. In at least some known blades, an abrupt transition extends between the root portion and the airfoil portion to increase the volume of cooling air entering the airfoil portion.
Gas turbine blades frequently have metallic surface coatings that are capable of resisting the oxidation, corrosion and sulfidation conditions generated during high temperature operation. Such coatings facilitate the airfoil withstanding thermal stresses that may be induced within the higher operating temperature areas of the blade. However, if the coating is applied at too great a thickness on regions of the blade operating at lower temperatures, such as the root and shank region, the combination of the increased coating thickness and the abrupt transition within the dovetail may cause cracking in the root portion as higher stresses are induced into the transition area of the dovetail. Over time, continued operation may lead to a premature failure of the blade within the engine.
The above coatings can be applied by depositing a vapor of one or more protective metals, for example aluminum or alloys of aluminum, on blade surfaces at high temperatures within a coating container or chamber commonly referred to as a “retort”. Generally, the blades to be coated are placed within the container, along with a source of the aluminide coating, typically in the form of metallic pellets retained in perforated baskets arranged in rows surrounding the blades. The coating container is then placed within a heater such as a furnace to generate an aluminide coating vapor. Generation of the coating vapor typically includes the use of halide “activators” such as fluorides, chlorides or bromides. The halide activator can be in the form of a gas that is introduced into the container to react with the source of the aluminide coating and form an aluminide-bearing gas, or it can be generated from a halide activator source within the container that forms a reactive halide gas upon heating.
While the above processes can be used to form an aluminide coating on the exterior and interior surfaces of turbine blades, processes using halide containing vapors at high deposition temperatures can be expensive and difficult to control, and may result in distortion of the blades, grain growth, creep and other thermo-mechanical failure mechanisms that can decrease the strength and life of the blade. Accordingly, it is desirable to provide a low-temperature, low-cost process to form a controlled, relatively uniform aluminide coating on the external and internal surfaces of turbine blades and other turbine components. It is also desirable that the aluminide coating formed on internal surfaces of turbine blades, particularly in the root and shank region, be relatively thin (for example, having a thickness of from about 0.0005 to about 0.0015 inches) (from about 12.7 to about 38.1 microns) so as not to cause premature cracking in the root portion of the blade.